The papermaking industry utilizes various amounts of recycled fiber or papers as a source of paper fiber furnish in the production of finished paper products. These recycled papers are often contaminated with pressure sensitive or hot melt adhesive tapes, seam bindings, labels, decals, stamps and stickers (e.g., bumper). These adhesives are referred to as stickies in the papermaking art.
Stickies are a diverse mixture of synthetic polymeric organic materials which are generally found in the application of recycled fiber papermaking processes. Stickies are relatively larger particles with a size in the range of several microns to several hundred microns. Pitch is a natural resinous substance which consists of resin/fatty acids and exists in the virgin fibers. White pitch occurs in paper mill recycling systems using broke with a latex coating binder. The size of pitch or white pitch is very small, and usually appears as colloidal particles.
During the pulping process, stickies are liberated from recycled fibers due to mechanical and thermal energy. For recycled fiber furnishes which contain a large quantity of plastic tapes, such as old corrugated containers, a significant amount of stickies are released during pulping. These stickies will not disperse well in the water and will either ultimately end up as "dirt spots" on the paper sheets or stick onto the wires, felts or other equipment requiring shutting down the manufacturing equipment in order to remove the stickies by solvent washing techniques.
According to the criteria developed by M. Doshi of Doshi & Associates, stickies can be classified on the basis of size:
______________________________________ Large, retained on 35 mesh screen, &gt;0.42 mm Medium, retained on 200 mesh screen, 0.074-0.42 mm Small, passed through 200 mesh screen, &lt;0.074 mm ______________________________________
Large stickies are relatively easily removed by cleaning and screening and therefore do not pose significant problems. Stickies with small size are generally less of a problem as long as they remain stable in the stock. Medium stickies are more likely to be a problem because they cannot efficiently be removed from the stock by a mechanical cleaning system. These stickies have a tendency to agglomerate and cause wire deposition or are a detriment to product quality.
Several non-chemical approaches which have been used in the paper industry to combat stickies include furnish selection, screening and cleaning, and thermal/mechanical dispersion units. Each of these approaches has limitations. Furnish selection may reduce but not completely eliminate the stickie contaminants in the system. It also increases the production cost significantly. Stickies cannot be completely removed with mechanical cleaning equipment since minimum slot size is limited to 0.15 mm for manufacturing reasons. Thermal/mechanical dispersion units can enhance breakup of stickies, but cannot prevent the agglomeration downstream, where problems usually occur. In addition, this approach requires capital investment and energy expenditure.
Chemical treatment can considerably reduce or alleviate stickies problems at a lower cost although its application technology may prove slightly complicated. Current chemical treatment techniques for stickies removal/control include detackification (passivation), dispersion, wire treatment (electrochemical control), maximization of solids retention, and chemical enhanced stickies removal during mechanical cleaning.